Тоже нужна проверка на ошибки Windows

#include <windows.h>
#include <stdlib.h>
#include <iostream> 
#include <fstream>
#include <iomanip>
using namespace std;
const int NN=100;
HANDLE hf;
HGLOBAL HN;
BYTE * buffer,*poss;
double was,now,timegone,speed;
DWORD size,cs,RValue,rest;
BYTE ch;
void main()
{
size=(256+rand()%100)*1024;
HN=GlobalAlloc(GMEM_MOVEABLE,size);
buffer=(BYTE*)GlobalLock(HN);
for (DWORD i=0;i<size;i++)
    {
    buffer[i]=rand()%256;
    }
RValue=512;
cout<<"Block size(b)|speed(Kb/s)\n";
ofstream gr("graph.txt");
gr<<"Block size(b)|speed(Kb/s)\n";
while (RValue<=1024)
    {
 
    hf = CreateFile(TEXT("result.dat"),GENERIC_WRITE,0,NULL,CREATE_ALWAYS,FILE_ATTRIBUTE_NORMAL,0);
    was=GetTickCount();
    for (int i=1;i<=NN;i++)
        {
        rest=size;
        poss=buffer;
        SetFilePointer(hf,0,NULL,FILE_BEGIN);
        while (rest>=RValue)
            {
            WriteFile(hf,poss,RValue,&cs,NULL);
            poss+=RValue;
            rest-=RValue;
            }
        WriteFile(hf,poss,rest,&cs,NULL);
        }  
  CloseHandle(hf);
  now=GetTickCount();
  timegone=(now-was)/   1000.;
  speed=size*NN/1024./timegone;
 
  cout << setw(5)<<RValue<<setw(15)<<setprecision(5)<<speed<<'\n';
  gr<< setw(5)<<RValue<<setw(15)<<setprecision(5)<<speed<<'\n';
  RValue+=32;
    }
GlobalUnlock(HN);
GlobalFree(HN);
gr.close();
cin.get();
}